Efforts to improve upon the performance of natural mineral oil based lubricants by the synthesis of oligomeric hydrocarbon fluids have been the subject of important research and development in the petroleum industry for at least fifty years and have led to the relatively recent market introduction of a number of superior polyalpha-olefin synthetic lubricants, primarily based on the oligomerization of alpha-olefins or 1-alkenes. In terms of lubricant property improvement, the thrust of the industrial research effort on synthetic lubricants has been toward fluids exhibiting useful viscosities over a wide range of temperature, i.e., improved viscosity index, while also showing lubricity, thermal and oxidative stability and pour point equal to or better than mineral oil. These new synthetic lubricants lower friction and hence increase mechanical efficiency across the full spectrum of mechanical loads in all types of gear boxes and do so over a wider range of operating conditions than mineral oil lubricants. In the prior art, oligomers of 1-alkenes from C.sub.6 to C.sub.20 have been prepared with commercially useful synthetic lubricants from 1-decene oligomerization yielding a distinctly superior lubricant product via either cationic or Ziegler catalyzed polymerization.
One characteristic of the molecular structure of 1-alkene oligomers that has been found to correlate very well with improved lubricant properties in commercial synthetic lubricants is the ratio of methyl to methylene groups in the oligomer. The ratio is called the branch ratio and is calculated from infra red data as discussed in "Standard Hydrocarbons of High Molecular Weight", Analytical Chemistry, Vol.25, no.10, p.1466 (1953). Viscosity index has been found to increase with lower branch ratio. Heretofore, oligomeric liquid lubricants exhibiting very low branch ratios have not been synthesized from 1-alkenes. For instance, oligomers prepared from 1-decene by either cationic polymerization or Ziegler catalyst polymerization have branch ratios of greater than 0.20. Shubkin, Ind. Eng.-Chem. Prod. Res. Dev. 1980, 19, 15-19, provides an explanation for the apparently limiting value for branch ratio based on a cationic polymerization reaction mechanism involving rearrangement to produce branching. Other explanations suggest isomerization of the olefinic group in the one position to produce an internal olefin as the cause for branching. Whether by rearrangement, isomerization or a yet to be elucidated mechanism it is clear that in the art of 1-alkene oligomerization to produce synthetic lubricants as practiced to-date excessive branching occurs and constrains the limits of achievable lubricant properties, particularly with respect to viscosity index. Obviously, increased branching increases the number of isomers in the oligomer mixture, orienting the composition away from the structure which would be preferred from a consideration of the concepts discussed above.
Recently, novel lubricant compositions (referred to herein as HVI-PAO) comprising polyalpha-olefins and methods for their preparation employing as catalyst reduced chromium on a silica support have been disclosed in U.S. patent applications Ser. No. 210,434, now U.S. Pat. No. 4,827,073, and 210,435, now U.S. Pat. No. 4,827,064 filed June 23, 1988, incorporated herein by reference. These novel lubricants exhibit high viscosity indices with low pour point. The compositions are characterized by a uniform molecular structure with low branch ratios comprising C.sub.30 -C.sub.1300 hydrocarbons with a branch ratio of less than 0.19. High yields are achieved in the oligomerization process for novel lubricant oligomers with viscosities above 20 cS at 100.degree. C. However, to produce lubricant with viscosity between 2 and 20 cS directly from the oligomerization of 1-alkene, high oligomerization temperatures are required. This results in both earlier catalyst deactivation and an increase in the yield of those oligomeric species, such as dimers of 1-alkenes, whose molecular weight is too low for lubricant use. Accordingly, the desirable, lubricant range low viscosity oligomer fraction is produced in relatively low yield and shortened catalyst life, compromising the economic viability of the direct synthesis route for low viscosity HVI-PAO product.
It is an object of the present invention to provide a method for the preparation of low viscosity range HVI-PAO in high yield.
It is another object of the present invention to provide a method for the preparation of low viscosity range HVI-PAO in high yield while retaining the high viscosity index and pour point characteristics of the novel oligomer.
Yet another object of the present invention is to prepare low viscosity HVI-PAO from high viscosity HVI-PAO.